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I've always wondered what the purpose of these "bars" on the nose of the F-16 (and some other military aircraft) are. Are they sensors (if so, what kind)? Are they lightning strips? I can't seem to figure out how to word my search query to find an answer, so I figured I'd ask here!

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Source: af.mil

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  • $\begingroup$ What are "lightning strips" that you hypothesize? All google turns up are decorative or sanding products. Did you possibly mean "lighting"? Like landing lights that are visible from forward, but don't shine into the pilot's eyes? $\endgroup$
    – Peter Cordes
    Mar 13, 2021 at 5:38
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    $\begingroup$ @PeterCordes What's being referred to there are discussed here: aviation.stackexchange.com/q/25020/7532. Obviously that isn't the correct answer, but as a first guess it's a reasonable one. $\endgroup$
    – Ralph J ♦
    Mar 13, 2021 at 17:05

2 Answers 2

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These are antennae for the IFF (Identification Friend or Foe) system. IFF is a system to help distinguish friendly aircraft from unfriendly ones.

This particular antenna arrangement is used on only a couple of F-16 models. The first was the F-16 Air Defense Fighter (ADF) version. This model was converted from the F-16 Block 15 for use as a fighter interceptor for the US Air National Guard. A total of 271 F-16A and F-16B aircraft were upgraded to this standard between 1989 and 1992. One of the modifications made during the conversion process was installation of the Teledyne/E Systems Mk.XII Advanced IFF system, given the designation APX-109. This system included four antennas, often nicknamed "bird slicers," placed in front of the cockpit canopy with additional antennas on the bottom of the inlet.

From: http://www.aerospaceweb.org/question/planes/q0286b.shtml

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Also, to add to Wayne's answer, there are multiple antennas because the computer that processes the data received compares the phase of the signals received from each of them to calculate the direction the signal came from... Because they are separated from one another, any signal received from a point directly in front of them will be received at the same time (it travels the same distance), and will therefore be in phase with the signals from the others.
But if the source is off to one side, then the signal received from the near antenna will have traveled a shorter distance, and will be out of phase with the others by an amount proportional to the sine of the angle to the source.

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  • $\begingroup$ (+1) It's interesting that it can detect the time difference for a signal traveling at the speed of light in that short of a distance! Do you know if there's also a vertical offset to detect signals from sources that are higher or lower than the F-16? Just because the nose is curved, it seems like there's an offset... but I can't tell if it's intentional. $\endgroup$
    – F16Falcon
    Mar 13, 2021 at 17:22
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    $\begingroup$ @F16Falcon: They are probably not measuring “arrival time” with a highly accurate clock (you’d probably need dozens of GHz) but instead just detect phase difference by subtracting the signals. $\endgroup$
    – Michael
    Mar 13, 2021 at 17:37
  • $\begingroup$ As @F16Falcon said, there is no clock involved. The processor simply adds, (or subtracts), the signals to measure the phase difference between them. Knowing the frequency of the signal, and that the two signals originated at the same point and time, the phase difference can be used to calculate the difference in distance traveled. Knowing the difference in distance traveled, and the geometry of the antennas, you can calculate the arrival angle. $\endgroup$
    – Charles Bretana
    Mar 15, 2021 at 1:58
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    $\begingroup$ As to whether the system detects the vertical angle, I flew the F-4, not the F-16, so I am not sure, but my assumption would be no, the main purpose of these systems (on the F-4 anyway), was to display the relative azimuth of threats on a circular threat display. In the F-4, during the Vietnam War, this was primarily ground threats like acquisition and tracking radars, and Active/Semi-Active Surface to Air Missiles (SAMs), so the vertical angle was not that important. $\endgroup$
    – Charles Bretana
    Mar 15, 2021 at 2:07

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